1. Field of the Invention
The present relates to transmission controls, and, in particular, controls for overriding an operator-selected transmission position.
2. Description of the Related Art
It is well known to control the engagement of a transmission in a vehicle hydraulically. This is particularly common in work vehicles, e.g., agricultural, forestry and industrial equipment, where the vehicle has numerous other hydraulically operated features.
In such a system, the gear shift lever moved by the vehicle operator changes the position of a hydraulic valve. The valve then regulates the hydraulic fluid connections to the activating valves in the transmission.
It is theoretically possible for an operator using such a system to turn the vehicle off with the selector in a gear. This means that when the vehicle is subsequently reactivated, it may lurch forward unexpectedly. Accordingly, it has been known for some time to include some form of engagement override valve, or EOV. The EOV overrides the transmission selection to prevent power from flowing to the drive shaft of the vehicle when the vehicle is started in any position other than neutral. An example of such a system is shown in U.S. Pat. No. 4,396,099 (Shirley).
It also is well known to provide a parking brake for a vehicle to hold it in position after the power is shut off. On work vehicles, such brakes frequently are spring-activated and hydraulically deactivated. This means that whenever the vehicle power is turned off or otherwise fails, the hydraulic pressure fails, and the parking brake is activated.
This default activation mode has the disadvantage that an operator may not realize the parking brake is on when he restarts the vehicle. Particularly in low gears, the vehicle may be able to drive through the parking brake. This can completely destroy the parking brake, and may result in damage to the transmission.